1. Field of the Invention
This invention relates to a method of fixing a toner image, which is formed on a supporting member by developing a latent image formed by an electrophotographic method, an electrostatic printing method or the like by a toner, on the supporting member.
2. Description of the Prior Art
A variety of fixing apparatuses have been proposed in the past to fix the toner image. Among them, a contact heating type fixing apparatus consisting, for example, of a rotatable fixing heat roller with a built-in heat source and a pressure contact roller rotating in the same direction as the heat roller while keeping contact with the heat roller has gained a wide practical application because the apparatus can provide a high heat efficiency but is free from the risk of fire.
Though the contact heat fixing system is preferable in various aspects as described above, it involves the problem that the toner attaches to the heat roller and then attaches once again to the subsequent supporting member supporting thereon the toner image to contaminate the fixed image. Accordingly, the toner attaching to the heat roller must be cleaned by use of cleaning members such as cleaning webs, cleaning blades, cleaning rollers, and the like.
When these cleaning members are disposed, however, so-called "back contamination" occurs. When excessive heat is applied to the toner material deposited on the cleaning member, the toner material is transferred to the heat roller, then contaminates the surface of the supporting member such as transfer paper that is subsequently fed, is further transferred to the pressure contact roller that is contacted to the heat roller and thus contaminates the back of the supporting member.
The inventors of the present invention have examined the cause of occurrence of this back contamination and have found that the elasticity of the toner at the time of fusing plays an important role. A thermoplastic resin consisting principally of a styrene-acrylic copolymer or a polyester resin has been believed preferable conventionally as a binder resin of the toner because it has a low softening point and provides high fixability. The elasticity improving compound is presented on the circumferential surface of the fixing roller other than at a point where the fixing roller is in press contact with the toner image supporting member. When fused, however, the resin has low elasticity and hence, is likely to transfer. Even though the resin is once collected by the cleaning member, therefore, the resin deposited on the cleaning member transfers to the heat roller when heated to a temperature which is by 10.degree. to 60.degree. C. higher than the set temperature of the heat roller after the heater of the heat roller is actuated, and eventually causes back contamination. If the elasticity of the toner at the time of fusing is adjusted in advance lest back contamination occurs, fixability drops and fixing can not be effected sufficiently.